We have recently found that some (but not all) lymphocytes from many mammalian species, including man, bind antigen-antibody-complement (AgAbC) complexes to their membranes. These lymphocytes, which we named CRL, have distinct properties and a characteristic distribution in lymphoid organs. The presence of multiple binding sites for AgAbC on the membrane of CRL suggests that immune complexes play a role in the life history of these cells. The experiments proposed in this research plan, in conjunction with the previously obtained findings, will serve to characterize CRL and to provide more information regarding their origin and function. In addition, they will clarify the relationship between CRL, theta-bearing lymphocytes, and the two functionally distinct populations of cells: thymus-derived and thymus-independent lymphocytes. The new information should validate the use of certain membrane markers to isolate and distinguish lymphocyte populations and provide a solid basis for their use as a tool to study the cellular events of the immune response.